Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Unexpected CPU behavior and the CLR instruction
Message-ID: <1990May30.184531.329@utzoo.uucp>
Organization: U of Toronto Zoology
References: <7101@ganymede.inmos.co.uk>
Date: Wed, 30 May 90 18:45:31 GMT

In article <7101@ganymede.inmos.co.uk> davidb@inmos.co.uk (David Boreham) writes:
>I think that the reason why the 68K and '11 do a read/modify/write
>on clear instructions is because they use the XOR ALU function to
>perform the clear...

I can't answer for the 68k, but on the 11 end:  "XOR?  What's that?".
The early 11s didn't have an XOR at all.

Many 11s did r-m-w on CLR simply because all the other single-operand
instructions in that category needed r-m-w, and it saved microcode space
and time to do them all the same way, with only the ALU operation chosen
specially for each one.
-- 
As a user I'll take speed over|     Henry Spencer at U of Toronto Zoology
features any day. -A.Tanenbaum| uunet!attcan!utzoo!henry henry@zoo.toronto.edu